Electronic detectors have long been used in photography and related fields to provide a measurement of the brightness of a scene or object. In order for the measurement to at least crudely represent the brightness as perceived by the human eye, detectors such as the cadmium sulfide photocell have been used. These detectors have spectral responsivities that peak in the visible region and at least roughly approximate the responsivity of the human eye. Such detectors, however, have characteristics that make them less than ideal for many uses.
More recently, optical filters have been used in combination with other detectors to provide a closer match to the human eye response.
In one approach, set forth in U.S. Pat. No. 3,996,461 (Sulzbach et al.), a multilayer thin film optical filter is deposited directly on the detecting surface of a silicon photodiode. The individual dielectric layers of the multilayer filter are deposited one at a time (on at least 50 silicon slices, each slice containing approximately 300 detectors) until an interference stack is built up. The multilayer filter is designed to reduce the light reaching the photodiode as a function of wavelength so that the detector system (photodiode with multilayer filter) has a spectral response close to that of the human eye. Because the silicon photodiode by itself has a spectral response weighted towards the red in the visible region but that continues to increase well into the infrared region, the multilayer filter reduces light transmission in both the infrared region and the visible region to yield the desired system response.
In another approach, phosphate glass-based filters containing copper ions are used as filters for the detectors. One drawback to these systems is the vulnerability of the phosphate glass to moisture. Another is the inconvenience and/or difficulty in processing the glass in molding, cutting, and polishing operations, as well as the relatively large specific gravity of the glass. Glass filters also tend to be quite thick and heavy, which is not desirable for many applications.
In other approaches, synthetic resin-based filters are used in place of glass-based filters For example, Japanese patent publications JP 06-118,228 and JP 06-345,877 disclose an optical filter made of synthetic resin consisting of a copolymer copolymerized from a mixture of a monomer containing phosphoric acid group of a specific structure and a monomer capable of being copolymerized with it. The filter also includes a metal salt mainly composed of copper salt. The phosphorous containing monomer has a phosphoric acid ester bond. The phosphoric acid-group causes the polymer to have poor weather resistance. As a result, if such an optical filter is exposed to high temperature and high humidity, problems relating to whitening (turbidity) and loss of transparency (opacification) begin to develop.
Other resin-based filters have also been proposed. Japanese patent publications 2000-98130 and 2000-252482 disclose an optical filter with improved durability by use of a polymer with a specially designed chemical structure. Such filters unfortunately have poor absorption of light in the near infrared region and the ultraviolet region. Detector systems using such filters therefore are sensitive to light that is not perceived by the human eye.
There is a continuing need for alternative detector systems that can simulate the human eye response, particularly systems having good out-of-band rejection (i.e., negligible response in near infrared and ultraviolet wavelengths), a good match to the desired response in the visible, and good weather resistance.